The invention relates to a method of predistorting data to compensate for the impulse response of a channel over which it is to be transmitted.
The invention further relates to a fixed wireless access system comprising an access point and a plurality of subscriber units. The invention still further relates to an access point and to a subscriber unit for use in such a system.
In radio communications systems, multipath interference is a major obstacle to high speed data transmission. Multipath interference arises when the radio signal is reflected and diffracted by objects in its path such as buildings, trees and even vehicles. Multipath interference causes echos from a particular data symbol to overlap with neighbouring symbols and this is known as intersymbol interference. Some form of equalisation is necessary to remove the intersymbol interference from the received signal. Unfortunately the equalisation process requires complicated signal processing and for high speed real time data communication systems this can be a significant problem.
In a fixed wireless access communications network, many subscriber units communicate with a single access point using a time division multiple access protocol. In a point to multipoint fixed wireless access system, each subscriber unit only receives data from a single access point and downlink bursts can often be heard by all subscriber units. Consequently, each subscriber unit has plenty of time to train its equaliser on the downstream channel. On the upstream channel however, signal transmissions occur between many different subscriber units and a single access point and so the access point has a very heavy equalisation burden as the channel characteristics of the different channels from the subscriber units to the access point may all be different and need different equalisation characteristics. As a result the equaliser at the access point will be required to adjust its coefficients rapidly to deal with bursts arriving from different subscriber units over channels having different impulse responses while the equaliser at each subscriber unit has the relatively simple task of equalising the single channel between itself and the access point. This is particularly significant when a frequency division duplex system is employed as there is no correlation between the upstream and downstream channels. During normal data transmission upstream data slots are scheduled by the access point which then knows the identity of the subscriber unit which is to transmit and therefore has prior knowledge of the impulse response of the channel between the scheduled subscriber unit and the access point. It is possible to use this prior knowledge to initialise the equaliser coefficients at the access point using a look up table so reducing training time and improving efficiency, assuming that the channel impulse response is unchanging or changes only slowly.
In order to obtain transmission time, each subscriber unit must first contend for channel access. This is achieved by transmitting a unique subscriber unit identifier, called a contention word, to the access point during a known contention field. The access point decodes the subscriber unit identifier and allocates a field in a subsequent data frame to that subscriber unit for upstream data transmission. The contention word transmitted from the subscriber unit is distorted by multipath interference due to reflection and diffraction in the radio channel and since the subscriber units have different physical positions the paths between the subscriber units and the access point are different leading to the channel distortion being different for each subscriber unit. Consequently, the received contention signal must be equalised before it can be decoded. During this contention procedure, however, the access point does not know the identity of the subscriber unit which is transmitting during a particular contention slot. This means that the access point has no prior knowledge of the channel impulse response and so the access point must retrain its equaliser for each contention burst. This retraining introduces delays, requires increased transmission overhead for an equaliser training sequence, and requires high computational complexity at the access point.
It is known that a precoder can be used to invert the impulse response of a communications channel, also known as the channel impulse response. The precoder predistorts the data signal using the inverse of the channel response so that the received signal (after passing through the channel) is free from distortion. One possibility is to use a linear finite impulse response (FIR) filter resulting in a linear precoder. The problem with a linear precoder is that any roots of the channel impulse response lying close to the unit circle will result in a very long FIR filter which may not be practically realisable. It is also possible to construct a precoder using a feedback or infinite impulse response (IIR) filter to invert the channel response. The problem with using an IIR filter is that the precoder is prone to instability.
A contention scheme disclosed in our copending UK Patent Application No: 0106604.2 (42559) relies on full precoding of the contention word at the subscriber unit and has the capability of removing a small number of critical zeros from the channel impulse response. It uses a linear precoder to cancel all roots of the channel impulse response except for those roots lying on the unit circle in the z plane. A root rotation method combined with pulse position modulation is used to remove critical zeros (zeros on the unit circle) from the channel response. In effect the input data word is modified to cancel out those zeros of the channel impulse response which the precoder is unable to cancel. The problem with this method is that some radio channels contain multiple critical zeros which are beyond the capacity of the root rotation method to remove.
The invention provides a fixed wireless access communications system comprising an access point and a plurality of subscriber units each transmitting a predetermined data sequence; in which each subscriber unit comprises a precoder for predistorting the predetermined data sequence to compensate for the characteristics of the upstream transmission channel between the subscriber unit and the access point wherein the system comprises means for optimising the precoder characteristic specifically for the predetermined data sequence.
Such an optimised preceding scheme is suited to the contention process in a fixed wireless access communication system where each subscriber unit wishes to transmit a short predetermined sequence known as a contention word during the contention slot. Typically, each subscriber unit only needs to transmit a single fixed sequence for contention rather than a number of arbitrary data sequences. Consequently, it is possible to optimise the precoder specifically for this sequence. This gives the advantage that the contention word can usually be ideally precoded before transmission either by using a short precoder or by storing the predistorted predetermined data sequence at the transmitter. A further advantage is that a linear precoder can be used avoiding the stability problems associated with non-linear precoders. Optimising the precoder for a unique data sequence allows more degrees of freedom for the precoder coefficients, so that a linear precoder can be selected that avoids roots lying near the unit circle in the z-plane. In a conventional linear precoder roots lying close to the unit circle in the z-plane result in a very long precoder. In addition, the length of the precoder is minimised which prevents excessive smearing of the received contention word in time, so minimising transmission overheads. In the second embodiment only a small memory capacity is required to store the predistorted contention word.
The precoder may be optimised using the recursive least squares algorithm. When using this algorithm the initial state of the signal correlation matrix P0 may be set to P0=δI, where δ>0.1σ2 and σ2 is the variance of the data samples, and where I is the identity matrix. The number of iterations may be restricted to be not substantially greater than the length of the precoder. The length of the precoder may be made equal to the length of the predetermined data sequence.
This gives the advantage that by using a relatively small number of iterations the computational complexity and processing time are minimised while the preceding obtained is optimised for the actual data sequence being transmitted by setting the initial parameters using a knowledge of the sequence to be received and decoded.
Generally the recursive least squares (RLS) algorithm used to train an equaliser employs a large number of iterations and sets the value of δ to a very much lower value, typically <0.001σ2. This ensures that the initial conditions do not significantly affect the tap coefficients produced and the equaliser or the precoder is trained for all data sequences. When the constraints given above are applied to the RLS algorithm the initial conditions do affect the tap co-efficients and the precoder is optimised specifically for the predetermined sequence.
Clearly alternative algorithms for obtaining precoder (or equaliser) tap coefficients could be used to implement the invention provided that they are constrained to optimise the precoder for the specific data sequence. Examples of alternative algorithms which could be used include gradient search methods.
A frequency division duplex, time division multiplex protocol may be used for communication between the access point and the subscriber units.
Precoding of transmissions from subscriber units is particularly advantageous in such systems as the characteristics of the upstream channels between the subscriber units and the access point are not correlated with those of the downstream channels between the access point and the subscriber units. This increases the processing load at the access point as each transmission from the subscriber units has to be equalised if preceding is not used and during contention the access point has no prior knowledge of the upstream channel characteristics as it does not know which subscriber unit is transmitting until it has decoded the contention word.
The invention further provides a method of pre-distorting a predetermined data sequence to compensate for the impulse response of a channel over which the predetermined data sequence is to be transmitted comprising the steps of;                transmitting the predetermined data sequence without precoding over the channel using a first transmitter,        receiving the predetermined data sequence using a first receiver comprising an equaliser and equalising the received signal, using an algorithm that is constrained to optimise the equaliser specifically for the predetermined sequence, to enable the sequence to be decoded;        determining the equaliser coefficients required to enable the equaliser to equalise the received data sequence,        applying the determined equaliser coefficients to a second transmitter;        transmitting the equaliser coefficients to a second receiver using the second transmitter,        receiving the equaliser coefficients at the second receiver, and        loading the received equaliser coefficients into a precoder in the first transmitter when the predetermined sequence is subsequently transmitted so that it is received at the first receiver in a form suitable for decoding without equalisation at the first receiver.        
The method may be modified by replacing the loading step by the steps of forming the predistorted predetermined data sequence and storing it in memory, and subsequently transmitting the stored predetermined data sequence so that it is received at the first receiver in a form suitable for decoding without equalisation at the first receiver.
By constraining the algorithm to optimise the equaliser for the predetermined sequence a linear equaliser of relatively short length can be used to enable the data sequence to be decoded and the precoder, which takes a similar form to the equaliser, is also of short length minimising smearing of the transmitted data sequence. Also, particularly when using the modified RLS algorithm to calculate the equaliser tap coefficients a relatively low computational complexity is involved and the reduced number of iterations result in a short processing time.
The first receiver and second transmitter may be located in the access point and the second receiver and first transmitter may be located in a subscriber unit wherein in order to set up the precoder to precode the contention word the following steps are implemented;                the access point is arranged to transmit a data field comprising a subscriber unit identifier, a test contention request control code, a contention word to be returned by the subscriber unit, and a test contention delay which indicates a reserved time slot during which the subscriber unit should transmit the test contention word,        the subscriber unit is arranged to receive and decode the transmitted data field and to transmit the received contention word without precoding to the access point in the reserved time slot,        the access point is arranged to receive the contention word, to train a linear equaliser using the received contention word, and to determined the equaliser tap coefficients,        the access point is arranged to transmit to the subscriber unit a data field comprising the subscriber unit identifier, the determined equaliser tap coefficients, and a flag indicating that the test contention word has been successfully decoded, and        the subscriber unit is arranged to apply the received tap coefficients to a linear precoder to predistort the contention word on subsequent transmission of the contention word to the access point.        
Alternatively the subscriber unit may be arranged to calculate and store a predistorted version of the contention word for subsequent transmission of the contention word to the access point.
The first contention word transmitted by the subscriber unit is a test contention word that is not precoded and is transmitted in a reserved contention slot, that is reserved for the specific subscriber unit. This first contention word is used to determine the precoder coefficients using an equaliser at the access point that has the same structure as the precoder at the subscriber unit. All subsequent contention words are precoded before transmission and transmitted during a non-exclusive contention slot that is also used by other subscriber units for contention.
This allows, in a system according to the invention, for the contention word to be allocated to a subscriber unit by the access point, for the precoder tap coefficients to be calculated at the access point and transmitted to the subscriber unit and for the subscriber unit to subsequently precode the contention word whenever it wishes to contend for access to a transmission slot between itself and the access point.
The invention still further provides an access point for use in a system according to the invention, the access point comprising a linear equaliser for equalising a predetermined data sequence received over a transmission channel, wherein the equaliser is optimised specifically to equalise the predetermined data sequence.
The access point may comprise a control unit for implementing the algorithm used to train the equaliser and determining the equaliser tap coefficients and a transmitter for transmitting the determined equaliser tap coefficients to the subscriber unit that transmitted the predetermined data sequence to the access point. The predetermined data sequence may be a contention word, wherein the control unit is arranged to allocate a contention word to be transmitted by a subscriber unit, to cause the allocated contention word to be transmitted to a selected subscriber unit together with an instruction to the subscriber unit to transmit the allocated contention word without preceding at a given time, and to train the equaliser using the received contention word at the given time.
The invention yet further provides a subscriber unit for use in a system according to the invention, the subscriber unit comprising a transmitter for transmitting a predetermined data sequence over a transmission channel and a precoder for preceding the predetermined data sequence to compensate for the impulse response of the transmission channel, wherein the precoder is optimised specifically to precode the predetermined data sequence.
The precoder may be a linear finite impulse response filter. Alternatively the precoder may comprise a memory in which a predistorted version of the predetermined data sequence is stored.
The subscriber unit may comprise a receiver for receiving data transmissions from an access point, a decoder for decoding the received data transmissions, a control unit for interpreting the data transmissions from the access point and controlling the response thereto of the subscriber unit, and a transmitter for transmitting data sequences to the access point; wherein the control unit is arranged to cause the subscriber unit to transmit a received contention word to the access point in response to an instruction received from the access point without precoding at a time specified by the access point and to apply preceding to the contention word on transmissions of the contention word subsequent to receiving precoder tap coefficients from the access point.